Electrodialysis is a well known art (see U.S. Pat. Nos. 4,325,792; 3,481,851; 3,909,381; 4,006,067; 3,983,016; 3,788,959; 3,926,759; 4,049,519; 4,057,483; 4,111,772; 4,025,405; 4,358,545; 3,793,163; 4,253,929; 4,325,798 and 4,439,293, the disclosures of which are incorporated by reference). Electrodialysis is the transport of ions through ion permeable membranes as a result of an electrical driving force. The process is commonly carried out in an electrochemical cell having a catholyte compartment containing a cathode and a catholyte and an anolyte compartment containing an anode and an anolyte, the catholyte and anolyte compartments being separated by ion permeable membranes. There is always in every electrodialysis process some small degree of reverse migration of cations through the anion permeable membrane and/or anions through the cation permeable membrane. Prior processes do not provide a satisfactory solution to the problem of reverse migration of hydroxyl ions through cation permeable membranes when the aqueous feed of the electrolysis cell contains multivalent metal cations. The hydroxyl ions reverse migrating react with multivalent cations being transported through the membrane to form substantially water insoluble salts in and on the cation permeable membranes. These salts reduce cell capacity at a constant voltage.
Acids are used broadly in the chemical, electronics, mining, electroplating and metal finishing industries wherein the acids react with metals and other salts to form salts of multivalent cations and anions of the respective acids. Prior processes do not provide a satisfactory method for regenerating and purifying acidic solutions containing multivalent cations and recovery of the multivalent metal cations as a substantially water insoluble salt. When electrolysis of aqueous solutions of salts of multivalent cations is carried out with a catholyte comprising hydroxyl ions to precipitate the multivalent cations that are transported through a cation permeable membrane, the hydroxyl ions tend to reverse migrate through the membrane and tend to form hydroxide precipitates of multivalent of multivalent cations in and on the membrane which foul the membranes. Considerably better results are obtained by the use of aqueous solutions of inorganic carbonate or bicarbonate as disclosed in my U.S. Pat. Nos. 4,325,792 and 4,439,293. When acidic catholytes are used in the electrodialysis of aqueous solutions of salts of multivalent cations, the cations are transported from an acidic solution through a cation permeable membrane into an acidic solution. The multivalent cations tend to electro deposit as metals in the membrane and on the cathode which require frequent maintenance of the cell. An object of this invention is to provide a high capacity electrodialytic process which can be used for continuous conversion of salts of multivalent cations into the acids and halogens of the anions of the salts and the hydroxides and other substantially water insoluble salts of the multivalent cations.
In the past decade, the chlor-alkali industry has focused its attention on developing membrane cells to produce low salt or salt-free caustic. Membranes have been developed for this purpose which are hydraulically impermeable but which will permit sodium ions to be transported from a brine anolyte while substantially preventing transport of chloride ions. Such cells are operated by flowing a brine solution into the anolyte compartment of an electrolysis cell and by providing salt-free water to the catholyte compartment to serve as the caustic medium, the anolyte compartment being separated from the catholyte compartment by a cation permeable membrane.
Membranes are now available that permit the manufacture of about 30 wt. % caustic soda, that is essentially free of salt, at a current efficiency of 90%. All of these membranes, however, are affected by impurities entering the cell with the incoming salt feed. An "ultra-pure" brine feed to the anolyte compartment of the cell is necessary for satisfactory performance (D. J. Gasser and R. J. Horvath paper presented at The Chlorine Insitute's 27th Chlorine Plant Operations Seminar, Washington, D.C., February 1984). Multivalent cations in the brine result in higher cell voltage and lower current efficiencies. In addition calcium and magnesium impurities are known to have a harmful effect on the useful life of membranes (Charles J. Molnar and Martin M. Dorio. Effects of brine quality on chlor-alkali membrane cell performance, 152nd National Meeting The Electrochemical Society, Atlanta, Ga., October 1977).
To meet performance and economic goals with membrane chlor-alkali processes, the industry has developed methods for purification of the brine. Gasser and Horvath in the above mentioned publication report that acceptable membrane cell performance can only be achieved with brine containing less than 50 ppb total hardness. By using ion-exchange in series with conventional primary treatments they report that an "ultra-pure" brine can be achieved.
In a closed loop membrane cell brine system, saturated brine flows through a primary treatment and secondary ion-exchange treatment, then to the membrane cells. Depleted brine, about 15 wt. % sodium chloride, from the cell is transferred to a dechlorination system where available and free chlorine are removed from the brine. Further chemical treatment using sodium sulfite is required to decompose residual chlorine, which if allowed to remain in solution would destroy the calcium adsorption capacity of the ion exchange resin. The sulfite treated brine is then saturated with salt and returned to the brine purification step. The addition of sulfite to the brine results in the formation of sulfate ions which are detrimental to cell performance.
The requirement for "ultra-pure" brine is a significant capital investment and operating cost for membrane processes. The brine purification system must be operated without upset to prevent damage to the membranes by multivalent cation salts. The dechlorination and chlorine decomposition steps must be operated without upset to prevent damage to the ion-exchange resins.